Automated Voice Processing Systems (VPS) have become prevalent over the past several years. Such systems typically include means for automatically answering a telephone and presenting the caller with a plurality of menu choices, any one or more of which may be selected by entering dual tone multi-frequency (DTMF) digits. The digits are detected and processed by one or more software applications. The different menu choices allow the user to achieve functions such as retrieving information, transferring bank balances, recording voice messages to be transmitted to others, and numerous other such applications.
Typically, a user is presented with a plurality of choices in the form of an audible menu. The user selects a desired choice by pushing predetermined DTMF tones.
It can be appreciated that the accurate detection of DTMF tones is critical to providing a reliable voice processing system. For example, if the DTMF detector incorrectly decodes a DTMF tone which has been entered, then the system may enter some undesirable mode or may even hang up on the caller. Additionally, during any time that the remote caller is speaking to the VPS, the caller's voice may simulate a DTMF tone, thereby causing the VPS to take the action associated with such DTMF tone being simulated. This phenomenon is termed talk off.
Finally, when the VPS is playing a message back to the caller, such message may be echoed back and interpreted by the DTMF detector as a valid DTMF tone. This is termed play off in the industry. Both talk off and play off may cause the system to enter some undesirable mode.
Numerous signal processing algorithms are presently commercially available for detecting DTMF tones. Most if not all of the algorithms take into account certain characteristics of the DTMF tones in order to determine and decode these tones. Such characteristics include, for example, average tone energy, signal-to-noise (SNR) ratio, twist, and one or more other parameters.
Twist is a parameter utilized by telephone engineers to define the difference in amplitude levels between the two frequencies that are contained within the DTMF tone. In the United States, the two frequencies are generated at equal levels, but the frequency response of the telephone network often causes one of the frequencies to be attenuated more than the other. Thus, if this difference in attenuation were 1 db, the DTMF signal that arrives at the receiver is said to have a twist of 1 db. In certain foreign countries, the DTMF signal is initially generated with a predetermined twist (e.g.; 2 db), and may arrive at the receiver with a different amount of twist.
A problem with analyzing these characteristics and therefore with detection of the DTMF digits, is that they often vary greatly from call to call. For example, average tone energy may vary by as much as 40 db from one call to the next. Thus, the portion of the DTMF detector which accounts for average tone energy must presume that if the tone energy is anywhere within a 40 db range, that a valid DTMF tone is present. This determination does not mean that a valid DTMF tone will be detected because other characteristics (e.g.; frequency, twist, etc.) must also meet the detection criteria before the system will conclude that a valid tone is present. These other characteristics of the tone, which include signal-to-noise ratio, twist, etc., may vary over large ranges from telephone call to telephone call.
The fact that these detection characteristics vary so greatly means that these systems are susceptible to phenomena such as talk off and play off. If the ranges within which some or all of these parameters are required to be for detection could be narrowed, then talk off and play off would occur less often.
Specifically, consider twist as a particular signal characteristic utilized in DTMF detection. The VPS is configured so that if the twist is within a predetermined range, then the DTMF detector presumes that a valid DTMF tone may have produced this twist. The larger the acceptable range, the more likely it is that voice will have characteristics which fall within that range, and that therefore, talk off will occur.
In view of this problem, it can be seen that there exists a need in the art to provide added protection against the phenomena of play off and talk off.